The invention relates to brush wear indicators for use with dynamoelectric machines and, more particularly, relates to a brush wear indicator that employs a brush-biasing spring as an active switch element for controlling a brush wear signaling circuit.
The use of brushes in dynamoelectric machines to conduct current from a rotating element, such as a commutator or slip ring, to a fixed electrical circuit is well known. In order to keep such circuits functioning properly, it is necessary to assure that the brushes are in good condition and are biased against the rotating conducting surface with a predetermined force. To accomplish these important objectives in most prior art dynamoelectric machines, it is necessary to periodically inspect the condition of the brushes in order to make sure that they are replaced before the brushes become so completely worn that they either fail to engage the rotating conductive commutator or slip ring with sufficient pressure to properly conduct current, or to make sure that neither the brush-biasing spring, nor a conductor embedded in the brush, engages the rotating commutator or slip ring thereby causing damage to it. Such required periodic inspections are frequently inconvenient to perform due to the relative inaccessibility of the brushes and associated brush holders, and in any event pose a nuisance to users of the different machines.
In recognition of those problems, several different types of brush wear indicators have already been developed. For example in U.S. Pat. No. 2,691,114-Lykins, which issued on Oct. 5, 1964, there is disclosed a torsion-spring arrangement for biasing a brush-follower arm against the outer end of a brush to hold it in engagement with a rotating conductive surface. The spring-biased arm is also effective to move a switch contact arm against a fixed switch contact when a predetermined degree of brush wear has occurred, thereby allowing the rigid brush-follower arm and its associated movable switch contact to move downward by a predetermined distance into contact with the fixed electrical contact of the switch. Responsive to closure of the switch, an associated signal is energized to alert maintenance personnel, or an operator of the machine, that the predetermined degree of brush wear has taken place. Another type of brush wear indicating mechanism is shown in U.S. Pat. No. 3,523,288-Thompson, which issued on Aug. 7, 1970. The indicating system disclosed in that patent comprises a spring-biased pin that is mounted in sliding relationship with one side of a carbon brush so that as the brush wears the pin is moved over a recess in the side of the brush and drops into the recess causing a signal circuit switch to be closed with a snap action. Again, closure of the switch alerts an operator to the predetermined degree of brush wear. This patent discloses a signaling circuit in which a normally closed switch is used to energize a signal means, such as a lamp, when the brush is in excess of a preselected given length. When the brush is worn to decrease its length below the preselected length, the normally closed switch is opened thereby extinguishing the signal lamp.
A patent quite similar to the above-noted Thompson patent is U.S. Pat. No. 4,121,270-Jones, which issued on Oct. 17, 1972 and discloses a brush wear indicator having an enclosed microswitch in combination with a switch actuating arm that rolls on one side of a brush. When the brush wears to a predetermined length, the switch-actuating arm rolls off the outer end of the brush and causes the switch to be closed with a snap action. In still another U.S. Pat. No. 4,172,988-Lowther, which issued on Oct. 30, 1979, there is disclosed a brush wear indicator in which an electrical contact mounted on the outer end of a brush is moved into engagement with a fixed contact mounted on an insulating member in the path of travel of the brush-mounted contact. In operation, when the brush is moved downward by a self-winding, brush-biasing spring, responsive to the brush being worn away by its sliding relationship with an associated slip ring or commutator on which it rides, the brush-mounted contact is moved into engagement with the fixed contact and completes a circuit, thereby energizing an indicating signal that alerts an operator to the predetermined degree of brush wear. Finally, in a co-pending U.S. Patent application having Ser. No. 183,920, there is disclosed a brush wear indicator that was conceived after the invention described herein, and that utilizes a self-winding, brush-biasing spring as an active switching element of a brush wear indicating assembly. Specifically, a coiled portion of the spring is operable to roll into engagement with either a flexible contact mounted adjacent a fixed end of the spring, or into actuating relationship with a switch mounted in the path of travel of the coiled portion of the spring, thereby to complete a circuit that energizes an indicating means to signal an operator of a predetermined degree of brush wear.
All of the brush wear indicator devices described in the patents and patent application mentioned above both differ from the invention described herein in that they either require the use of auxiliary switch actuating means, or switches, in addition to the spring brush-biasing mechanism of the associated brush holder assembly, or they require specialized, flexible contacts that can be mounted on and engaged by a self-winding, brush-biasing spring, such as that disclosed in the Michael application. Accordingly, it is a primary object of the present invention to provide a brush wear indicator for dynamoelectric machines that is characterized by providing a reliable and commercially feasible brush wear sensor and associated indicating circuit that utilizes a self-winding, brush-biasing spring as an active switch element of the indicating circuit, in combination with a simple printed circuit board type of fixed contact arrangement to avoid the relatively greater costs of prior art brush wear indicators while achieving desired advantages of greater reliability and lower manufacturing cost.
Another object of the invention is to provide a brush wear indicator that utilizes a conductive self-winding, brush-biasing spring as an electrically conductive element that effectively completes an electrical circuit between at least two contacts of an associated indicating circuit when a brush mounted in a holder adjacent the spring is in a relatively unworn condition, but is also operable to open the indicating circuit responsive to the brush being worn sufficiently to allow the biasing spring to move out of engagement with one of the fixed contacts.
Still another object of the invention is to provide a brush wear indicator that includes a printed circuit board type of indicating circuit element that readily affords an indication of a plurality of different degrees of brush wear, without requiring the use of a plurality of separately actuated movable switching elements.
Further objects and advantages of the invention will become apparent to those skilled in the art from the description of it that follows considered in conjunction with the accompanying drawings.